Controllers & Action Methods

1. What is a Controller?

A controller is a class that:

• Handles HTTP requests
• Calls the data/service layer
• Returns a response (view, JSON, file, status code, redirect, etc.)

Think: request coordinator

Rules

• Class name ends with Controller
• Inherits from Controller (MVC) or ControllerBase (API)
• Lives in the Controllers folder (by convention)

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2. Action Methods

Action methods are:

• public methods inside controllers
• Each handles one request
• Selected via routing

They usually return IActionResult.

public IActionResult Index()
{
    return Content("Hello");
}

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3. Attribute Routing (Most Common)

Routes are defined directly on controllers/actions.

[Route("home")]
public class HomeController : Controller
{
    [HttpGet("")]
    public IActionResult Index() => Content("Home");

    [HttpGet("about")]
    public IActionResult About() => Content("About");
}

Enable controllers:

builder.Services.AddControllers();
app.MapControllers();

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4. IActionResult (Key Concept)

IActionResult lets an action return different responses based on logic.

One action → many possible results

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5. Common Action Results (Know These)

ContentResult (raw text / HTML)

return Content("<h1>Hello</h1>", "text/html");

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JsonResult (JSON response)

return Json(new { id = 1, name = "Book" });

(Content-Type auto = application/json)

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File Results

return File("/sample.pdf", "application/pdf");          // from wwwroot
return PhysicalFile(@"C:\file.pdf", "application/pdf");
return File(bytes, "application/pdf");                  // in-memory

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Status Code Results

return Ok(); // 200
return BadRequest("Invalid"); // 400
return NotFound(); // 404
return Unauthorized(); // 401
return StatusCode(403); // custom

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Redirect Results

return Redirect("/home"); // URL
return RedirectToAction("Index", "Home"); // action
return LocalRedirect("/products/10"); // local only

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6. Typical Action with Validation

[HttpGet("book")]
public IActionResult GetBook(int id)
{
    if (id <= 0)
        return BadRequest("Invalid id");

    if (id > 1000)
        return NotFound();

    return File("/sample.pdf", "application/pdf");
}

One action, multiple IActionResults

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7. When to Use What (Mental Map)

Need	Use
Plain text / HTML	Content()
API data	Json()
File download	File()
Validation error	BadRequest()
Missing resource	NotFound()
Success	Ok()
Navigate elsewhere	Redirect*()

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8. Interview One-Liner 🎯

"Controllers group related actions that handle HTTP requests. Each action returns an IActionResult, which allows flexible responses like JSON, files, redirects, or status codes. Routing decides which action runs."